As a catalyst for purifying exhaust gas from an automobile, the use of a beta zeolite which has been ion-exchanged with iron ions has been proposed (refer to Patent Documents 1 to 3). For example, Patent Document 1 discloses a denitration catalyst having a carrier where a beta zeolite having a SiO2/Al2O3 mol ratio of 15 to 300 has been ion-exchanged with 0.1 to 15 mass % of Fe3+ ions, and ferric oxide supported on this carrier.
Patent Document 2 discloses that an exhaust gas which contains a nitrogen oxide is contacted with a beta zeolite having a framework structure where the Si content percentage attributed to the Q4 of the zeolite framework measured by the 29Si MAS NMR spectrum is 35 to 47 mass %, and the mol ratio of SiO2/Al2O3 is at least 20 and less than 100, and which has been subjected to ion exchange and supports Fe3+.
Patent Document 3 discloses a production method of a NOx adsorbent. This method includes an impregnation step of impregnating a beta zeolite with an iron chloride aqueous solution to make an iron chloride-containing zeolite, an ion exchange step of ion exchanging Fe by heating the iron chloride containing zeolite at 330° C. to 500° C. under a moisture-free atmosphere, and a heat treatment step of heat treating the iron chloride-containing zeolite after the ion exchange step under a non-oxidizing atmosphere.
Besides these techniques, a hydrocarbon reforming/trapping material has been proposed with the objectives of increasing the purifying efficiency of the hydrocarbon by a three-way catalyst which is a purification catalyst for exhaust gas from an automobile, and economizing the use of noble metals, where for a hydrocarbon temporarily captured inside a zeolite micropore having a metal oxide, the complete purification temperature of the hydrocarbon is reduced by reforming the hydrocarbon to a chemical species with higher reactivity, with the result that it becomes possible to economize the use of noble metals (refer to Patent Document 4 and Non-Patent Document 1).
Patent Document 1: Pamphlet of PCT International Publication No. WO2006/011575
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2007-076990
Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2008-264702
Patent Document 4: Japanese Unexamined Patent Application, Publication No. 2008-073625
Non-Patent Document 1: M. Ogura, T. Okubo, and S. P. Elangovan, Catal. Lett., 118, 72(2007).